The requirement exists in many fields of application for measuring surfaces of objects and therefore also the objects themselves with high precision. This is true in particular for the manufacturing industry, for which the measuring and checking of surfaces of workpieces has high significance. A variety of approaches exist for this purpose, which extend from contacting methods up to optical sensors. In the field of high-precision optical methods, interferometric measuring principles, in particular in conjunction with the use of coordinate measuring devices, play an increasing role.
One possibility is the use of white-light interferometry for high-precision measuring. In this case, the utilization is either scanning, i.e., by adjusting the interferometer, and therefore slowly or with spectrally resolved detection, typically with restriction to a measuring range of a few millimeters. The field of use of such arrangements is therefore restricted and in particular workpieces having a strongly structured surface and correspondingly varying measuring distances cannot be measured or can only be measured with severe restrictions, for example, long travel times.
Other methods use a frequency-modulated laser beam as measuring radiation for an interferometric arrangement. Thus, for example, an approach is known from WO 2009/036861 A1, in which in a method for measuring surfaces, a frequency-modulated laser beam is generated and emitted onto the surface to be measured. After the measuring radiation reception of the backscattered from the surface as the target, the distance is determined by interferometry, wherein a measuring interferometer arm and a reference interferometer arm having a partially shared beam path are used. Deviations from the essentially perpendicular incidence of the measuring radiation on the surface in the case of distance measurements are taken into consideration by an algorithm or avoided or reduced during the scanning guiding by control of the emission of the measuring radiation.
The partially shared beam path of measuring interferometer arm and reference interferometer arm is delimited in this case by a reflection within the optical measuring head, which thus defines the reference interferometer. This so-called common path architecture permits the local oscillator plane to be arranged within the measuring head optic, for example, also on the optical exit surface, and therefore close to the target. The advantage of this construction is that environmental influences, for example, temperature changes or vibrations, act in the same way on both interferometer arms, so that the generated signals are subject to the same influences in this regard. However, one disadvantage of the construction is the requirement of a long coherence length, if a sufficient signal strength within the operating range is to be ensured.
In contrast, external interferometer arms having adjustable delay are used in the field of white-light interferometry. However, other boundary conditions are also provided for the common field of application of medical technology. Thus, these structures to be scanned or measured are fundamentally different in type and structured less with regard to the distances. In addition, absolute distance information is not necessary and the time scales typically required for a measurement are less than in the case of measurements of industrial parts. As a result of the surfaces to be measured, longer measuring durations are required here and as a result of the geometries to be measured, greater measuring ranges are typically also necessary. Solutions of this prior art are found, for example, in US 2004/061865, US 2008/117436, or DE 198 19 762, which describe white-light interferometers, which do not have a tunable laser source for generating frequency-modulated laser radiation. In U.S. Pat. No. 4,627,731, the division of the light signal upstream of the measuring interferometer into two paths having modulators of different frequencies is used to generate a heterodyne frequency. This so-called modulation interferometer also requires precise equalization of the path lengths.
In the case of interferometric measuring arrangements using frequency-modulated laser radiation, however, the measuring range is delimited by the coherence length thereof, so that the field of application is subject to restrictions and corresponding expenditure is required on the control side to be able to scan and measure a measuring object completely and in a short time.
One problem is to provide an improved measuring method or measuring arrangement, respectively, for measuring surfaces or for detecting surface topographies.
A further problem is to provide such a measuring method or a measuring arrangement, respectively, which overcomes the restrictions existing due to the coherence length and therefore increases the measurable distance range.